Trench-assisted multimode optical fiber

ABSTRACT

A trench-assisted, multimode optical fiber includes a central core having an alpha refractive index profile with respect to an outer cladding. The optical fiber also includes an inner cladding, a depressed trench, and an outer cladding. The optical fiber achieves reduced bending losses and a high bandwidth.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application hereby claims the benefit of U.S. Patent ApplicationNo. 61/539,346 for a Trench-Assisted Multimode Optical Fiber (filed Sep.26, 2011), which is hereby incorporated by reference in its entirety.

This application is continuation-in-part of U.S. patent application Ser.No. 12/878,449 (Molin et al.) for a Multimode Optical Fiber HavingImproved Bending Losses (filed Sep. 9, 2010, and published Mar. 10,2011, as U.S. Patent Publication No. US2011/0058781 A1), which itselfclaims the benefit of U.S. Patent Application No. 61/241,592 for a FibreOptique Multimode Présentant des Pertes en Courbure Améliorées (filedSep. 11, 2009) and French application Ser. No. 09/04305 for a FibreOptique Multimode Présentant des Pertes en Courbure Améliorées (filedSep. 9, 2009, at the National Institute of Industrial Property(France)). French application Ser. No. 09/04305, U.S. Patent ApplicationNo. 61/241,592, U.S. patent application Ser. No. 12/878,449, and U.S.Patent Publication No. US2011/0058781 A1 are hereby incorporated byreference in their entirety.

This application is continuation-in-part of U.S. patent application Ser.No. 12/953,948 (Molin et al.) for a High-Bandwidth, Multimode OpticalFiber with Reduced Cladding Effect (filed Nov. 24, 2010, and publishedMay 26, 2011, as U.S. Patent Publication No. US2011/0123161 A1), whichitself claims the benefit of U.S. Patent Application No. 61/265,101 fora Fibre Optique Multimode à Très Large Bande Passante avec une InterfaceCœur-Gaine Optimiséé (filed Nov. 30, 2009) and French application Ser.No. 09/58381 for a Fibre Optique Multimode à Très Large Bande Passanteavec une Interface Cœur-Gaine Optimiséé (filed Nov. 25, 2009, at theNational Institute of Industrial Property (France)). French applicationSer. No. 09/58381, U.S. Patent Application No. 61/265,101, U.S. patentapplication Ser. No. 12/953,948, and U.S. Patent Publication No.US2011/0123161 A1 are hereby incorporated by reference in theirentirety.

This application is continuation-in-part of U.S. patent application Ser.No. 12/954,036 (Molin et al.) for a High-Bandwidth, Dual-Trench-AssistedMultimode Optical Fiber (filed Nov. 24, 2010, and published May 26,2011, as U.S. Patent Publication No. US2011/0123162 A1), which itselfclaims the benefit of U.S. Patent Application No. 61/265,115 for a FibreOptique Multimode à Très Large Bande Passante avec une InterfaceCœur-Gaine Optimiséé (filed Nov. 30, 2009) and French application Ser.No. 09/58382 for a Fibre Optique Multimode à Très Large Bande Passanteavec une Interface Cœur-Gaine Optimiséé (filed Nov. 25, 2009, at theNational Institute of Industrial Property (France)). French applicationSer. No. 09/58382, U.S. Patent Application No. 61/265,115, U.S. patentapplication Ser. No. 12/954,036, and U.S. Patent Publication No.US2011/0123162 A1 are hereby incorporated by reference in theirentirety.

This application is continuation-in-part of U.S. patent application Ser.No. 12/959,688 (Molin et al.) for a Multimode Optical Fiber with LowBending Losses and Reduced Cladding Effect (filed Dec. 3, 2010, andpublished Jun. 9, 2011, as U.S. Patent Publication No. US2011/0135262A1), which itself claims the benefit of U.S. Patent Application No.61/266,746 for a Fibre Optique Multimode à Large Bande Passante et àFaibles Pertes par Courbure (filed Dec. 4, 2009) and French applicationSer. No. 09/58637 for a Fibre Optique Multimode à Large Bande Passanteet à Faibles Pertes par Courbure (filed Dec. 3, 2009, at the NationalInstitute of Industrial Property (France)). French application Ser. No.09/58637, U.S. Patent Application No. 61/266,746, U.S. patentapplication Ser. No. 12/959,688, and U.S. Patent Publication No.US2011/0135262 A1 are hereby incorporated by reference in theirentirety.

This application is continuation-in-part of U.S. patent application Ser.No. 12/959,866 (Molin et al.) for a High-Bandwidth Multimode OpticalFiber Having Reduced Bending Losses (filed Dec. 3, 2010, and publishedJun. 9, 2011, as U.S. Patent Publication No. US2011/0135263 A1), whichitself claims the benefit of U.S. Patent Application No. 61/266,754 fora Fibre Optique Multimode à Large Bande Passante et à Faibles Pertes parCourbure (filed Dec. 4, 2009) and French application Ser. No. 09/58639for a Fibre Optique Multimode à Large Bande Passante et à Faibles Pertespar Courbure (filed Dec. 3, 2009, at the National Institute ofIndustrial Property (France)). French application Ser. No. 09/58639,U.S. Patent Application No. 61/266,754, U.S. patent application Ser. No.12/959,866, and U.S. Patent Publication No. US2011/0135263 A1 are herebyincorporated by reference in their entirety.

This application is continuation-in-part of U.S. patent application Ser.No. 13/037,943 (Molin et al.) for a Broad-Bandwidth Multimode OpticalFiber Having Reduced Bending Losses (filed Mar. 1, 2011, and publishedSep. 8, 2011, as U.S. Patent Publication No. US2011/0217012 A1), whichitself claims the benefit of U.S. Patent Application No. 61/323,619 fora Fibre Optique Multimode à Large Bande Passante et à Faibles Pertes parCourbure (filed Apr. 13, 2010) and French application Ser. No. 10/51489for a Fibre Optique Multimode à Large Bande Passante et à Faibles Pertespar Courbure (filed Mar. 2, 2010, at the National Institute ofIndustrial Property (France)). French application Ser. No. 10/51489,U.S. Patent Application No. 61/323,619, U.S. patent application Ser. No.13/037,943, and U.S. Patent Publication No. US2011/0217012 A1 are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTIONS

The present invention relates to the field of fiber optic transmissionand, more specifically, to a trench-assisted multimode optical fiberhaving reduced bending losses and a high bandwidth for high-data-rateapplications.

BACKGROUND

An optical fiber (i.e., a glass fiber typically surrounded by one ormore coating layers) conventionally includes an optical fiber core,which transmits and/or amplifies an optical signal, and an opticalcladding, which confines the optical signal within the core.Accordingly, the refractive index of the core n_(c) is typically greaterthan the refractive index of the optical cladding n_(g) (i.e.,n_(c)>n_(g)).

For optical fibers, the refractive index profile is generally classifiedaccording to the graphical appearance of the function that associatesthe refractive index with the radius of the optical fiber.Conventionally, the distance r to the center of the optical fiber isshown on the x-axis, and the difference between the refractive index (atradius r) and the refractive index of the optical fiber's outer cladding(e.g., an outer optical cladding) is shown on the y-axis. The refractiveindex profile is referred to as a “step” profile, a “trapezoidal”profile, a “parabolic” profile (e.g., an “alpha” profile), or a“triangular” profile for graphs having the respective shapes of a step,a trapezoid, a parabola, or a triangle. These curves are generallyrepresentative of the optical fiber's theoretical or set profile.Constraints in the manufacture of the optical fiber, however, may resultin a slightly different actual profile.

Generally speaking, two main categories of optical fibers exist:multimode fibers and single-mode fibers. In a multimode optical fiber,for a given wavelength, several optical modes are propagatedsimultaneously along the optical fiber. In a single-mode optical fiber,the signal propagates in a fundamental LP01 mode that is guided in theoptical-fiber core, while the higher order modes (e.g., the LP11 mode)are strongly attenuated. The typical diameter of a single-mode ormultimode glass fiber is 125 microns. The core of a multimode opticalfiber typically has a diameter of between about 50 microns and 62.5microns, whereas the core of a single-mode optical fiber typically has adiameter of between about 6 microns and 9 microns. Multimode systems aregenerally less expensive than single-mode systems, because multimodelight sources, connectors, and maintenance can be obtained at a lowercost.

Multimode optical fibers are commonly used for short-distanceapplications requiring a broad bandwidth, such as local networks or LAN(local area network). Multimode optical fibers have been the subject ofinternational standardization under the ITU-T G.651.1 recommendations,which, in particular, define criteria (e.g., bandwidth, numericalaperture, and core diameter) that relate to the requirements for opticalfiber compatibility. The ITU-T G.651.1 standard (July 2007) is herebyincorporated by reference in its entirety.

In addition, the OM3 standard has been adopted to meet the demands ofhigh-bandwidth applications (i.e., a data rate higher than 1 GbE) overlong distances (i.e., distances greater than 300 meters). The OM3standard is hereby incorporated by reference in its entirety. With thedevelopment of high-bandwidth applications, the average core diameterfor multimode optical fibers has been reduced from 62.5 microns to 50microns.

Typically, an optical fiber should have the broadest possible bandwidthto perform well in a high-bandwidth application. For a given wavelength,the bandwidth of an optical fiber may be characterized in severaldifferent ways. Typically, a distinction is made between the so-called“overfilled launch” condition (OFL) bandwidth and the so-called“effective modal bandwidth” condition (EMB). The acquisition of the OFLbandwidth assumes the use of a light source exhibiting uniformexcitation over the entire radial surface of the optical fiber (e.g.,using a laser diode or light emitting diode (LED)).

Recently developed light sources used in high-bandwidth applications,such as VCSELs (Vertical-Cavity Surface-Emitting Lasers), exhibit aninhomogeneous excitation over the radial surface of the optical fiber.For this kind of light source, the OFL bandwidth is a less suitablemeasurement, and so it is preferable to use the effective modalbandwidth (EMB). The calculated effective bandwidth (EMBc) estimates theminimum EMB of a multimode optical fiber independent of the kind ofVCSEL used. The EMBc is obtained from a differential-mode-delay (DMD)measurement (e.g., as set forth in the FOTP-220 standard).

An exemplary method of measuring DMD and calculating the effective modalbandwidth can be found in the FOTP-220 standard, which is herebyincorporated by reference in its entirety. Further details on thistechnique are set forth in the following publications, each of which ishereby incorporated by reference: P. F. Kolesar and D. J. Mazzarese,“Understanding Multimode Bandwidth and Differential Mode DelayMeasurements and Their Applications,” Proceedings of the 51st Int'l Wireand Cable Symposium, 2002, pp. 453-460; and Doug Coleman and PhillipBell, “Calculated EMB Enhances 10 GbE Performance Reliability forLaser-Optimized 50/125 μm Multimode Fiber,” Corning Cable SystemsWhitepaper (March 2005).

FIG. 1 shows a schematic diagram of a DMD measurement according to thecriteria of the FOTP-220 standard as published in its TIA SCFO-6.6version of Nov. 22, 2002. FIG. 1 schematically represents a part of anoptical fiber (i.e., an optical core surrounded by an outer cladding). ADMD graph is obtained by successively injecting into the multimodeoptical fiber a light pulse having a given wavelength λ₀ with a radialoffset between each successive pulse. The delay of each pulse is thenmeasured after a given length of fiber L. Multiple identical lightpulses (i.e., light pulses having the same amplitude, wavelength, andfrequency) are injected with different radial offsets with respect tothe center of the multimode optical fiber's core. The injected lightpulse is depicted in FIG. 1 as a black dot on the optical core of theoptical fiber. In order to characterize an optical fiber with a50-micron diameter, the FOTP-220 standard recommends that individualmeasurements be carried out at radial offset intervals of about twomicrons or less. From these measurements, it is possible to determinethe modal dispersion and the calculated effective modal bandwidth(EMBc).

The TIA-492AAAC-A standard, which is hereby incorporated by reference inits entirety, specifies the performance requirements for50-micron-diameter multimode optical fibers used over long distances inEthernet high-bandwidth transmission network applications. The OM3standard requires, at a wavelength of 850 nanometers, an EMB of at least2,000 MHz·km. The OM3 standard assures error-free transmissions for adata rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. The OM4standard requires, at a wavelength of 850 nanometers, an EMB of at least4,700 MHz·km to obtain error-free transmissions for a data rate of 10Gb/s (10 GbE) up to a distance of 400 meters. The OM4 standard is herebyincorporated by reference in its entirety.

In a multimode optical fiber, the difference between the propagationtimes, or group delay times, of the several modes along the opticalfiber determine the bandwidth of the optical fiber. In particular, forthe same propagation medium (i.e., in a step-index multimode opticalfiber), the different modes have different group delay times. Thisdifference in group delay times results in a time lag between the pulsespropagating along different radial offsets of the optical fiber.

For example, as shown in the graph on the right side of FIG. 1, a timelag is observed between the individual pulses. This FIG. 1 graph depictseach individual pulse in accordance with its radial offset in microns(y-axis) and the time in nanoseconds (x-axis) the pulse took to passalong a given length of the optical fiber.

As depicted in FIG. 1, the location of the peaks along the x-axisvaries, which indicates a time lag (i.e., a delay) between theindividual pulses. This delay causes a broadening of the resulting lightpulse. Broadening of the light pulse increases the risk of the pulsebeing superimposed onto a trailing pulse, which reduces the bandwidth(i.e., data rate) supported by the optical fiber. The bandwidth,therefore, is linked to the group delay time of the optical modespropagating in the multimode core of the optical fiber. Thus, toguarantee a broad bandwidth, it is desirable for the group delay timesof all the modes to be identical. Stated differently, the intermodaldispersion should be zero, or at least minimized, for a givenwavelength.

To reduce intermodal dispersion, the multimode optical fibers used intelecommunications generally have a core with a refractive index thatdecreases progressively from the center of the optical fiber to itsinterface with a cladding (i.e., an “alpha” core profile). Such anoptical fiber has been used for a number of years, and itscharacteristics have been described in “Multimode Theory of Graded-CoreFibers” by D. Gloge et al., Bell system Technical Journal 1973, pp.1563-1578, and summarized in “Comprehensive Theory of Dispersion inGraded-Index Optical Fibers” by G. Yabre, Journal of LightwaveTechnology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of theabove-referenced articles is hereby incorporated by reference in itsentirety.

A graded-index profile (i.e., an alpha-index profile) can be describedby a relationship between the refractive index value n and the distancer from the center of the optical fiber according to the followingequation:

$n = {n_{1}\sqrt{1 - {2{\Delta\left( \frac{r}{a} \right)}^{\alpha}}}}$

wherein,

α≧1, and α is a non-dimensional parameter that is indicative of theshape of the index profile;

n₁ is the maximum refractive index of the optical fiber's core;

a is the radius of the optical fiber's core; and

$\Delta = \frac{\left( {n_{1}^{2} - n_{0}^{2}} \right)}{2\; n_{1}^{2}}$

where n₀ is the minimum refractive index of the multimode core, whichmay correspond to the refractive index of the outer cladding (most oftenmade of silica).

A multimode optical fiber with a graded index (i.e., an alpha profile)therefore has a core profile with a rotational symmetry such that alongany radial direction of the optical fiber the value of the refractiveindex decreases continuously from the center of the optical fiber's coreto its periphery. When a multimode light signal propagates in such agraded-index core, the different optical modes experience differingpropagation mediums (i.e., because of the varying refractive indices).This, in turn, affects the propagation speed of each optical modedifferently. Thus, by adjusting the value of the parameter α, it ispossible to obtain a group delay time that is nearly equal for all ofthe modes. Stated differently, the refractive index profile can bemodified to reduce or even eliminate intermodal dispersion.

In practice, however, a manufactured multimode optical fiber has agraded-index central core surrounded by an outer cladding of constantrefractive index. The core-cladding interface interrupts the core'salpha-index profile. Consequently, the multimode optical fiber's corenever corresponds to a theoretically perfect alpha profile (i.e., thealpha set profile). The outer cladding accelerates the higher-ordermodes with respect to the lower-order modes. This phenomenon is known asthe “cladding effect.” In DMD measurements, the responses acquired forthe highest radial positions (i.e., nearest the outer cladding) exhibitmultiple pulses, which results in a temporal spreading of the responsesignal. Therefore, bandwidth is diminished by this cladding effect.

Multimode optical fibers are commonly used for short-distanceapplications requiring a high bandwidth, such as local area networks(LANs). In such applications, the optical fibers may be subjected toaccidental or otherwise unintended bending, which can give rise tosignal attenuation and modify the mode power distribution and thebandwidth of the optical fiber.

It is therefore desirable to achieve multimode optical fibers that areunaffected by bends having a radius of curvature of less than 10millimeters. One proposed solution involves adding a depressed trenchbetween the core and the cladding. Nevertheless, the position and thedepth of the trench can significantly affect the optical fiber'sbandwidth.

Therefore, a need exists for a multimode optical fiber having reducedbending losses and a high bandwidth.

SUMMARY

In one aspect, the present invention embraces a multimode optical fiberthat, for a length of 300 meters at a wavelength between about 840nanometers and 860 nanometers, the multimode optical fiber exhibits aleaky-mode bandwidth (EMB_(leaky)) of about 1,850 MHz-km or greater.

In another aspect, the present invention embraces a multimode opticalfiber that, for a length of 400 meters at a wavelength between about 840nanometers and 860 nanometers, the multimode optical fiber exhibits aleaky-mode bandwidth (EMB_(leaky)) of about 2,550 MHz-km or greater(e.g., at least about 3,880 MHz-km).

In this regard, one exemplary multimode optical fiber satisfying one orboth of these leaky-mode-bandwidth criteria includes a central coresurrounded by an outer cladding. The central core has an outer radius r₁and an alpha-index profile with respect to the outer cladding. An innercladding is positioned between the central core and the outer cladding(e.g., immediately surrounding the central core). The inner cladding has(i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive indexdifference Δn₂ with respect to the outer cladding. A depressed trench ispositioned between the inner cladding and the outer cladding (e.g.,immediately surrounding the inner cladding). The depressed trench has(i) an outer radius r_(t), (ii) a width w_(t), and (iii) a refractiveindex difference Δn_(t) with respect to the outer cladding.

In one exemplary embodiment, for two turns around a bend radius of 5millimeters at a wavelength of 850 nanometers, the multimode opticalfiber has bending losses of less than about 1 dB (e.g., less than about0.3 dB).

In another exemplary embodiment, for two turns around a bend radius of7.5 millimeters at a wavelength of 850 nanometers, the multimode opticalfiber has bending losses of less than about 0.4 dB (e.g., less thanabout 0.2 dB).

In yet another exemplary embodiment, for two turns around a bend radiusof 10 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 0.3 dB (e.g., lessthan about 0.1 dB).

In yet another exemplary embodiment, for two turns around a bend radiusof 15 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 0.1 dB (e.g., lessthan about 0.05 dB).

The foregoing illustrative summary, as well as other exemplaryobjectives and/or advantages of the invention, and the manner in whichthe same are accomplished, are further explained within the followingdetailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an exemplary DMD measurement method andgraph.

FIG. 2 graphically depicts the refractive index profile of an exemplaryoptical fiber according to the present invention.

FIG. 3 schematically depicts optical-ray propagation in a conventionalmultimode fiber with a graded-index core.

FIG. 4 schematically depicts optical-ray propagation in atrench-assisted multimode fiber with a graded-index core.

FIG. 5 schematically depicts the refractive index profile of anexemplary optical fiber and the modes propagating in the optical fiber.

FIG. 6 graphically depicts the real relative effective refractiveindices of the modes as a function of the leakage losses of the leakymodes in the optical fiber of FIG. 5.

FIG. 7 depicts (i) the refractive index, (ii) the effective refractiveindex n_(eff), and (iii) the field amplitude of a leaky mode as afunction of radial offset in an exemplary graded-index multimode opticalfiber including a depressed trench.

FIG. 8 depicts (i) the refractive index, (ii) the effective refractiveindex n_(eff), and (iii) the field amplitude of the highest-order guidedmode as a function of radial offset in a comparative graded-indexmultimode optical fiber.

FIGS. 9A and 9B depict graphs plotting system margin as a function ofboth effective modal bandwidth and spectral width for a given link atdifferent link lengths.

FIG. 10 depicts the transfer functions of the guided modes, the leakymodes, and the optical link for an exemplary optical fiber.

FIG. 11 depicts the encircled flux coordinates of the simulatedVertical-Cavity Surface-Emitting Lasers (VCSELs).

FIG. 12 depicts the distributions of insertion loss (IL) obtained withsimulated VCSEL sources of FIG. 11.

FIG. 13 depicts the distributions of power ratio between leaky modes andguided modes obtained with simulated VCSEL sources of FIG. 11.

FIG. 14 graphically depicts, for four bend-insensitive multimode opticalfibers, leakage losses of the leaky modes as a function of optical fiberlength.

FIG. 15 graphically depicts, for the four bend-insensitive multimodeoptical fibers of FIG. 14, leakage losses per meter of the leaky modesas a function of time delay per meter.

FIG. 16 graphically depicts, for the four bend-insensitive multimodeoptical fibers of FIGS. 14-15, the average time delay of each opticalfiber's set of leaky modes under over-filled launch conditions as afunction of optical fiber length.

FIG. 17 graphically depicts, for the four bend-insensitive multimodeoptical fibers of FIGS. 14-16 under over-filled launch conditions, theeffective modal bandwidth EMB_(leaky) of the leaky modes as a functionof optical fiber length.

FIG. 18 graphically depicts the effective modal bandwidth of an opticallink EMB_(link) as a function of the effective modal bandwidthEMB_(leaky) of the leaky modes.

FIG. 19 graphically depicts the effective modal bandwidth of an opticallink EMB_(link) as a function of the effective modal bandwidthEMB_(leaky) of the leaky modes.

DETAILED DESCRIPTION

The present invention embraces a multimode optical fiber that achievesreduced bending losses and a high bandwidth.

FIG. 2 depicts the refractive index profile of an exemplary opticalfiber in accordance with the present invention as disclosed in parentU.S. patent application Ser. No. 12/953,948, now published as U.S.Patent Publication No. US2011/0123161 A1.

The exemplary optical fiber depicted in FIG. 2 includes a central corehaving an outer radius r₁ and an alpha refractive index profile (i.e.,an alpha-index profile) with respect to an outer cladding (e.g., anouter optical cladding) surrounding the central core. Typically, thecore has an outer radius r₁ of about 25 microns (e.g., ±1.5 microns).The refractive index difference between the central core and the outercladding typically has a maximum value Δn₁ of between about 11×10⁻³ and16×10⁻³ (e.g., between about 12×10⁻³ and 15×10⁻³), such as a maximumrefractive index difference Δn₁ of about 1% (i.e., 14.5×10⁻³). Thecentral core typically has an alpha profile with an alpha parameter ofbetween about 1.9 and 2.1. In a particular embodiment, the central corehas an alpha profile with an alpha parameter of between about 2.04 and2.10, such as between about 2.06 and 2.09 (e.g., between about 2.07 and2.08). In another particular embodiment, the central core has an alphaprofile with an alpha parameter of between about 2.05 and 2.08 (e.g.,between about 2.06 and 2.07).

For reasons of cost, the outer cladding (i.e., the outer opticalcladding) is typically made of natural silica, but it may alternativelybe made of doped silica.

The exemplary optical fiber depicted in FIG. 2 includes an innercladding positioned between the central core and the outer cladding. Inone embodiment, the inner cladding immediately surrounds the centralcore. The inner cladding has an outer radius r₂, a width w₂, and arefractive index difference Δn₂ with respect to the outer cladding. Therefractive index difference Δn₂ between the inner cladding and the outercladding is typically between about −1.0×10⁻³ and 1.0×10⁻³ (e.g.,between about −0.5×10⁻³ and 0.5×10⁻³), more typically between about−0.2×10⁻³ and 0.2×10⁻³ (e.g., between about −0.05×10⁻³ and 0.05×10⁻³).More typically, and as illustrated in FIG. 2, the refractive indexdifference Δn₂ between the inner cladding and the outer cladding isapproximately zero. The inner cladding's width w₂ is typically less than5 microns, more typically less than 3 microns (e.g., between about 1micron and 2 microns, such as 1.5 microns). The characteristics of theinner cladding facilitate the achievement of high bandwidths.

The exemplary optical fiber depicted in FIG. 2 includes a depressedtrench positioned between the inner cladding and the outer cladding. Byway of example, the depressed trench may immediately surround the innercladding. The depressed trench has an outer radius r_(t), a width w_(t),and a refractive index difference Δn_(t) with respect to the outercladding. The width w_(t) of the depressed trench is typically betweenabout 3 and 5 microns (μm) (e.g., about 4 microns).

Typically, the term “depressed trench” is used to describe a radialportion of an optical fiber that has a refractive index that issubstantially less than the refractive index of the outer cladding. Inthis regard, the refractive index difference Δn_(t) between thedepressed trench and the outer cladding is typically between about−15×10⁻³ and −3×10⁻³, more typically between about −10×10⁻³ and −5×10⁻³(e.g., about −6×10⁻³).

Generally speaking, a refractive index difference with respect to theouter cladding can be expressed as a percentage using the followingequation:

${\Delta\mspace{14mu}\%(r)} = \frac{100 \times \left( {{n(r)}^{2} - n_{cladding}^{2}} \right)}{2{n(r)}^{2}}$where n(r) is the comparative refractive index value as a function ofradial position (e.g., the refractive index n_(t) of the depressedtrench), and n_(cladding) is the refractive index value of the outercladding. Those of ordinary skill in the art will appreciate that thisequation can be used if the refractive index varies over a given sectionof the optical fiber (i.e., the refractive index value varies as afunction of radial position) or if the refractive index is constant overa given section of the optical fiber.

Those of ordinary skill in the art will appreciate that the outercladding typically has a constant refractive index. That said, if theouter cladding has a non-constant refractive index, the refractive indexdifference (e.g., Δn₁, Δn₂, Δn_(t), Δ₁%, Δ₂%, or Δ_(t)%) between asection of the optical fiber and the outer cladding is measured usingthe refractive index of the inner most portion of the outer cladding(i.e., the portion of the outer cladding that is expected to moststrongly affect the propagation of optical signals within the opticalfiber).

A constant refractive index difference with respect to the outercladding can also be expressed as a percentage by the followingequation:

${\Delta\mspace{14mu}\%} = \frac{100 \times \left( {n^{2} - n_{cladding}^{2}} \right)}{2n^{2\;}}$where n is the comparative refractive index (e.g., the refractive indexn_(t) of the depressed trench), and n_(cladding) is the refractive indexof the outer cladding.

Parent U.S. patent application Ser. No. 12/953,948, now published asU.S. Patent Publication No. US2011/0123161 A1, discloses that therefractive index difference Δn_(t) between the depressed trench and theouter cladding and the width w_(t) of the depressed trench should behigh enough to facilitate low bending losses.

As disclosed in parent U.S. patent application Ser. No. 12/953,948, nowpublished as U.S. Patent Publication No. US2011/0123161 A1, a depressedtrench's volume v_(t) may be defined by the following equation:

v_(t) = 2π × ∫_(r_(int))^(r_(t))Δ_(t)  %(r) × r × 𝕕rin which Δ_(t)% (r) is the depressed trench's refractive indexdifference as a function of radial position with respect to the outercladding expressed in percentage, r_(t) is the outer radius of thedepressed trench, and r_(int) is the inner radius of the depressedtrench (e.g., the outer radius r₂ of the inner cladding). Those ofordinary skill in the art will appreciate that this equation can be usedif the depressed trench's refractive index difference varies (i.e., thetrench is non-rectangular) or if the refractive index difference isconstant (i.e., the depressed trench is rectangular).

If the refractive index difference between the depressed trench and theouter cladding is constant, the volume v_(t) of the depressed trench canalso be defined by the following simplified equation:v _(t)=Δ_(t)%·π·(r _(t) ² −r _(int) ²)in which Δ_(t)% is the depressed trench's refractive index differencewith respect to the outer cladding expressed in percentage, r_(t) is theouter radius of the depressed trench, and r_(int) is the inner radius ofthe depressed trench (e.g., the outer radius r₂ of the inner cladding).

For some embodiments of the exemplary optical fiber depicted in FIG. 2,the volume v_(t) of the depressed trench is typically between about200%·μm² and 1,200%·μm². More typically, the volume v_(t) of thedepressed trench is between about 250%·μm² and 750%·μm². Thecharacteristics of the depressed trench facilitate the achievement oflow bending losses.

Those having ordinary skill in the art will appreciate that theforegoing inclusion of a constant (e.g., pi) is not crucial tocharacterizing the size of a depressed trench. Parent U.S. patentapplication Ser. No. 12/878,449, now published as U.S. PatentPublication No. US2011/0058781 A1, discloses characterizingdepressed-trench volume with respect to the trench's width W_(t) andrefractive index difference Δn_(t) with the outer optical cladding,namely V=1000×w_(t)·Δn_(t). Parent U.S. patent application Ser. No.12/878,449 schematically depicts refractive index profiles of twomultimode optical fibers having (i) a central core with an outer radiusr₁ and an alpha index profile with respect to the outer optical claddingand (ii) a depressed trench having a constant width W_(t) (i.e., arectangular, step refractive index profile) and a refractive indexdifference Δn_(t) with the outer optical cladding. Parent U.S. patentapplication Ser. No. 12/878,449 also discloses an exemplary depressedtrench volume of less than about −40 microns (e.g., between about −20microns and −40 microns), typically between about −30 microns and −40microns, as defined by the equation V=1000×W_(t)×Δn_(t).

As further disclosed in parent U.S. patent application Ser. No.12/953,948, now published as U.S. Patent Publication No. US2011/0123161A1, the relation between the width w₂ of the inner cladding and therefractive index difference Δn_(t) of the depressed trench makes itpossible to achieve both low bending losses and a high bandwidth.

For example, at a wavelength of 850 nanometers, optical fibers inaccordance with the present invention typically have: (i) for two turnswith a bend radius (e.g., a radius of curvature) of 15 millimeters,bending losses of less than about 0.1 dB (e.g., less than about 0.05dB), (ii) for two turns with a bend radius of 10 millimeters, bendinglosses of less than about 0.3 dB (e.g., less than about 0.1 dB), (iii)for two turns with a bend radius of 7.5 millimeters, bending losses ofless than about 0.4 dB (e.g., less than about 0.2 dB), and (iv) for twoturns with a bend radius of 5 millimeters, bending losses of less thanabout 1 dB (e.g., less than about 0.3 dB).

Conventionally, guided light rays are guided by total internalreflection within a multimode optical fiber. See FIG. 3. For multimodeoptical fibers that include a depressed trench, however, the most angledlight rays, which are not reflected and thus lost in conventionaloptical fibers, are reflected back to the core at the interface of theinner cladding and the trench. This reflection is not total (i.e.,partial reflection) because of the finite trench width. As aconsequence, the most angled light rays lose energy (i.e., leak outreflection after reflection) during their propagation along the opticalfiber. See FIG. 4. The leakage losses of these leaky light rays (andthus of the leaky modes) depend on the radial width of the depressedtrench. Wider depressed trenches reduce the leakage losses of the leakymodes. In practice, however, the depressed trench has a finite width,barring total internal reflection and thereby resulting in some leakagelosses.

Nevertheless, leaky modes, like guided modes, are eigenmodes of thewaveguide. The leaky modes experience effective refractive indicesn_(eff) that are complex. In this regard, analysis of the imaginary partof the effective refractive indices n_(eff) provides informationregarding their leakage losses. For example, at a given wavelength λ,the leakage losses L_(leaky) of the leaky modes (dB/m) may be determinedusing the following equation:

$L_{leaky} = {{\frac{20}{\ln(10)} \cdot \frac{2\pi}{\lambda}}{{{Im}\left( n_{eff} \right)}.}}$

FIG. 5 schematically depicts the refractive index profile of anexemplary multimode optical fiber and the modes propagating in theoptical fiber. Additional propagation modes (i.e., leaky modes) areobserved below the baseline, zero value of the relative effectiverefractive index n_(eff) (i.e., with respect to the refractive index ofthe outer optical cladding). The real parts of the leaky modes'effective refractive indices (i.e., real (n_(eff))) are (i) lower thanthose of the guided modes and (ii) lower than the refractive index ofthe outer cladding.

FIG. 6 shows the relationship between the real part of the relativeeffective refractive indices n_(eff) of the leaky modes and simulatedleakage losses in decibels per meter (dB/m) experienced by the leakymodes in a multimode optical fiber having the refractive index profiledescribed in FIG. 5. As depicted, leaky modes with real part of lowereffective refractive indices (i.e., more negative) exhibit higherleakage losses.

FIG. 7 depicts (i) the real part of the refractive index, (ii) theeffective refractive index n_(eff), and (iii) the field amplitude as afunction of radial offset for a leaky mode in an exemplary graded-indexmultimode optical fiber that includes a depressed trench. FIG. 8 depicts(i) the refractive index, (ii) the effective refractive index n_(eff),and (iii) the field amplitude as a function of radial offset for thehighest-order guided mode of a comparative graded-index multimodeoptical fiber. The comparative multimode optical fiber of FIG. 8includes an outer cladding that is depressed to the same refractiveindex of the depressed trench in the exemplary optical fiber of FIG. 7(i.e., below LP_(12,7)). The effective refractive indices n_(eff) of theleaky modes in FIG. 7 have real parts (i.e., non-imaginary parts)similar to those of the highest-order guided modes FIG. 8.

FIGS. 9A and 9B depict system margin as a function of both effectivemodal bandwidth and spectral width for given links at different linklengths. System margin is shown (i) in grayscale according to the legendprovided on the right of FIGS. 9A and 9B and by (ii) the plotted contourlines.

The leftmost plot in FIG. 9A provides data for a link length of 30meters. As demonstrated, for link lengths of 30 meters or less, thelink's bandwidth is not a limiting factor for system margin providedthat the link's effective modal bandwidth exceeds 250 MHz-km.

The central and rightmost plots in FIG. 9A provide data for link lengthsof 100 meters and 200 meters, respectively. To achieve a margin of 0 dBat a 0.45-nanometer spectral width RMS, a 100-meter link should have aneffective modal bandwidth of at least 500 MHz-km. To achieve a margin of0 dB at a 0.45-nanometer spectral width RMS, a 200-meter link shouldhave an effective modal bandwidth of at least 1100 MHz-km.

The left and right plots in FIG. 9B provide data for link lengths of 300meters and 400 meters, respectively. To achieve a margin of 0 dB at a0.45-nanometer spectral width RMS, a 300-meter link should have aneffective modal bandwidth of at least 1,960 MHz-km. To achieve a marginof 0 dB at a 0.45-nanometer spectral width RMS, a 200-meter link shouldhave an effective modal bandwidth of at least 3,880 MHz-km.

For regular multimode optical fibers (i.e., multimode optical fibersthat are not designed for bend-insensitivity), the power of the lightsource coupled to the optical fiber is split into a guided-mode set witha limited effective modal bandwidth, thereby reducing system margins.Moreover, when the coupled power is split into the guided-mode set, afraction of the coupled power is typically lost, which leads toinsertion losses, thereby reducing system margins.

For a bend-insensitive multimode optical fiber having the same centralcore as a regular multimode optical fiber, the leaky modes catch a partof the coupled power that a regular multimode optical fiber would lose.In this regard (and as before), the power of the light source coupled tothe optical fiber is split into a guided-mode set with a limitedeffective modal bandwidth, thereby reducing system margins. When thecoupled power is split into the guided-mode set, a smaller fraction ofthe coupled power is lost than the percentage lost by a regularmultimode optical fiber, because, as noted, the leaky modes capture partof the coupled power. As compared with a regular multimode opticalfiber, a bend-insensitive multimode optical fiber typically has lowerinsertion losses and thus improved system margins. Furthermore, thecoupled power split into the leaky modes can actually increase systemmargins.

The bandwidth of a multimode optical fiber (e.g., a link) depends onboth the guided modes and the leaky modes. The following equations canbe used to calculate bandwidth, assuming perfect synchronization betweenthe leaky and the guided channels:

y(t) = (H_(guided)(t) + H_(leaky)(t)) ⊗ x(t) s_(i n)(t) = x²(t)s_(out)(t) = y²(t)${{{\overset{\sim}{H}}_{link}(f)}}^{2} = \frac{\begin{matrix}{{\mathbb{e}}^{- \frac{f^{2}}{\sigma_{guided}^{2}}} + {A^{2} \cdot {\mathbb{e}}^{- \frac{f^{2}}{\sigma_{leaky}^{2}}}} +} \\{{2 \cdot A \cdot {\mathbb{e}}^{{- {({\frac{1}{2 \cdot \sigma_{guided}^{2}} + \frac{1}{2 \cdot \sigma_{leaky}^{2}}})}} \cdot f^{2}} \cdot \cos}\;\Delta\;\Phi}\end{matrix}}{1 + A^{2} + {{2 \cdot A \cdot \cos}\;{\Delta\Phi}}}$where:

-   -   H_(guided)(t) is the transfer function of the guided modes;    -   H_(leaky)(t) is the transfer function of the leaky modes;    -   s_(in)(t) is the input signal;    -   s_(out)(t) is the output signal;    -   H_(link) f is the transfer function of the link accounting both        for leaky and guided modes;    -   σ_(guided) is proportional to the bandwidth of the        guided-mode-only channel;    -   σ_(leaky) (or EMB_(leaky)) is proportional to the bandwidth of        the leaky-mode-only channel;    -   A² is the ratio of leaky-mode power to guided-mode power; and    -   ΔΦ is the global phase shift between the guided modes and the        leaky modes, which depends on the average delay between the        guided modes and leaky modes.

FIG. 10 depicts the transfer functions of the guided modes, the leakymodes, and the optical link for an exemplary multimode optical fiber.For this exemplary optical link, the effective modal bandwidth of theguided modes EMB_(guided) is 10,500 MHz-km, and the effective modalbandwidth of the leaky modes (EMB_(leaky)) is 2,000 MHz-km. The ratio A²of leaky-mode power to guided-mode power is 16 percent, and the globalphase shift ΔΦ between the guided modes and the leaky modes is zero.These values yield an effective modal bandwidth of 4,700 MHz-km for thisexemplary optical link.

To estimate the ratio A² of leaky-mode power to guided-mode power, thepower coupled into the leaky modes can be assessed using insertion losscomputations expected in a regular multimode optical fiber underVertical-Cavity Surface-Emitting Laser (VCSEL) launches. In this regard,encircled flux (EF) and insertion loss modeling under random VCSELlaunches can be performed, for example, using the methods described byPepeljugoski et al. in “Modeling and Simulation of Next-GenerationMultimode Fiber Links,” Journal of Lightwave Technology, Vol. 21, No. 5(May 2003), which is hereby incorporated by reference in its entirety.

FIG. 11 depicts the encircled flux coordinates of the simulatedVertical-Cavity Surface-Emitting Lasers (VCSELs). FIG. 12 depicts thedistributions of insertion loss (IL) obtained with all of the simulatedVCSEL sources (i.e., “all”) and with only the simulated VCSEL sourcesthat fulfill the encircled flux (EF) requirements of IEEE 802.3 Standard(i.e., “EF within specs”). FIG. 13 depicts the distributions of powerratio between leaky modes and guided modes obtained with all of thesimulated VCSEL sources (i.e., “all”) and with only the simulated VCSELsources that fulfill the encircled flux (EF) requirements of IEEE 802.3Standard (i.e., “EF within specs”). The IEEE 802.3 Standard, which issometimes referred to as 10 GbE, is hereby incorporated by reference inits entirety. As shown by the data of FIGS. 11-13, for 98 percent of thelaunch conditions, the power of the leaky modes is less than 10 percentof the power of the guided modes after one meter of propagation (i.e.,assuming the trench-assisted optical fiber has substantially the samecore refractive-index profile as an otherwise identical conventionaloptical fiber).

FIG. 14 graphically depicts leakage losses of the leaky modes as afunction of optical fiber length. The four exemplary trench-assistedmultimode optical fibers have inner-cladding widths of 1.0 micron, 1.5microns (i.e., Draka's “MaxCap-BB”), 2.0 microns, and 3.0 microns,respectively, but are otherwise identical. As shown in FIG. 14, thepower of the leaky modes decreases as fiber length increases. After onemeter of propagation, the power of the leaky modes is attenuated byapproximately 8 dB.

FIG. 15 graphically depicts, for the four exemplary trench-assistedmultimode optical fibers of FIG. 14, leakage losses (dB/m) of the leakymodes as a function of time delay (ps/m). The two dark diamond plots andthe triangle plot represent data for comparative bend-insensitivemultimode optical fibers. The light diamond plot represents data for apreferred bend-insensitive multimode optical fiber (i.e., MaxCap-BB).Thus, the data of FIG. 15 demonstrates that leakage losses and timedelays for the leaky modes depend on the depressed trench's design.Additionally, the leaky modes with the greatest leakage losses exhibitthe largest time delays.

FIG. 16 graphically depicts, for the four exemplary trench-assistedmultimode optical fibers of FIGS. 14 and 15, the average time delay ofeach optical fiber's set of leaky modes under over-filled launchconditions as a function of optical fiber length. The uppermost plot andtwo lowest plots represent data for exemplary bend-insensitive multimodeoptical fibers. The plot with the relatively constant average time delayof slightly less than zero represents data for a preferredbend-insensitive multimode optical fiber (i.e., MaxCap-BB). In thisregard, the leaky modes of the preferred bend-insensitive multimodeoptical fiber have an average time delay that does not vary with opticalfiber length. Therefore, the average time delay of the leaky modes isboth relatively small and predictable. For this preferred multimodeoptical fiber under over-filled launch conditions, the average delaybetween the guided modes and the leaky modes rapidly tends to zero asoptical fiber length increases.

Using data from FIGS. 11-16, the ratio A² of leaky-mode power toguided-mode power and the global phase shift ΔΦ between the guided modesand the leaky modes can be estimated. These values can then be used tocalculate the effective modal bandwidth EMB_(leaky) of a given multimodeoptical fiber's leaky modes via the equations discussed previously. Inthis regard, FIG. 17 graphically depicts, for the four trench-assistedmultimode optical fibers of FIGS. 14-16 under over-filled launchconditions, the effective modal bandwidth EMB_(leaky) of the leaky modesas a function of optical fiber length. The uppermost plot representsdata for the preferred bend-insensitive multimode optical fiber (i.e.,MaxCap-BB), whereas the three lower plots represent data for the otherexemplary bend-insensitive multimode optical fibers.

For the preferred multimode optical fiber (i.e., MaxCap-BB), theeffective modal bandwidth EMB_(leaky) of the leaky modes increases withoptical fiber length. This increase in effective modal bandwidthEMB_(leaky) of the leaky modes occurs because, as shown in FIG. 15, theleakage losses of the leaky modes typically increase as time delaysincrease. In other words, the leaky modes with the greatest time delays,which decrease EMB_(leaky), experience greater leakage losses. As thesedelayed leaky modes propagate over the length of the optical fiber,their leakage losses increase, thereby reducing their detrimental effecton the optical fiber's effective modal bandwidth EMB_(leaky). Indeed,FIG. 17 demonstrates that, for optical fiber lengths greater than onemeter, the leaky modes of the exemplary multimode optical fiber (i.e.,MaxCap-BB) have an effective modal bandwidth EMB_(leaky) of greater thanabout 3,000 MHz-km (i.e., when initially all leaky modes are equallyexcited).

The leaky modes also impact the effective modal bandwidth of the opticallink EMB_(link). Assuming (i) a ratio A² of leaky-mode power toguided-mode power of 10 percent and (ii) the greatest global phase shiftΔΦ between the guided modes and the leaky modes for the preferredoptical fiber (i.e., MaxCap-BB), the leaky modes contribute anadditional 10 percent power to the optical link, which can increasesystem margin by 0.4 dB. For optical fiber lengths of 100 meters, 200meters, 300 meters, and 400 meters, exemplary multimode optical fibershave EMB_(link) values of at least about 500 MHz-km, 1,100 MHz-km, 1,960MHz-km, and 3,880 MHz-km, respectively.

FIGS. 18 and 19 graphically depict the effective modal bandwidth of anoptical link EMB_(link) as a function of the effective modal bandwidthEMB_(leaky) of the leaky modes. For FIG. 18, the effective modalbandwidth EMB_(guided) of the guided modes is set to 2,000 MHz-km (i.e.,the lowest OM3-compliant value), and A² is set to 10 percent. For thegraph of FIG. 19, the effective modal bandwidth EMB_(guided) of theguided modes is set to 4,700 MHz-km (i.e., the lowest OM4-compliantvalue), and A² is set to 10 percent. (In either case, the phase shift ΔΦis not necessary to the calculation.)

FIG. 18 demonstrates that a multimode optical fiber having a leaky-modeeffective modal bandwidth (EMB_(leaky)) of about 1,850 MHz-km or greaterprovides safe transmission with borderline OM3-compliant multimodeoptical fibers at 300 meters. FIG. 19 demonstrates that a multimodeoptical fiber having a leaky-mode effective modal bandwidth(EMB_(leaky)) of about 2,550 MHz-km or greater provides safetransmission with borderline OM4-compliant multimode optical fibers at400 meters.

An exemplary multimode optical fiber includes a central core surroundedby an outer cladding. (For reasons of cost, the outer cladding istypically made of natural silica, but alternatively it may be made ofdoped silica.) The central core has an outer radius r₁ and analpha-index profile with respect to the outer cladding. An innercladding is positioned between the central core and the outer cladding(e.g., immediately surrounding the central core). The inner cladding has(i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive indexdifference Δn₂ with respect to the outer cladding. A depressed trench ispositioned between the inner cladding and the outer cladding (e.g.,immediately surrounding the inner cladding). The depressed trench has(i) an outer radius r_(t), (ii) a width w_(t), and (iii) a refractiveindex difference Δn_(t) with respect to the outer cladding.

Typically, the central core has an outer radius r₁ of between about 23microns and 27 microns (e.g., 25 microns). The refractive indexdifference between the central core and the outer cladding typically hasa maximum value Δn₁ of between about 11×10⁻³ and 16×10⁻³ (e.g., betweenabout 12×10⁻³ and 15×10⁻³, such as about 14.5×10⁻³). The alpha-indexprofile of the central core has an alpha parameter α of between about2.0 and 2.1 (e.g., between about 2.04 and 2.09, such as 2.06 or so).

Typically, the inner cladding immediately surrounds the central core.The inner cladding has a width w₂ of between about 1 and 3 microns,typically about 1.5 microns. The refractive index difference Δn₂ betweenthe inner cladding and the outer cladding is typically between about−0.05×10⁻³ and 0.05×10⁻³. More typically, the refractive indexdifference Δn₂ between the inner cladding and the outer cladding isapproximately zero. The characteristics of the inner cladding facilitatethe achievement of high bandwidths.

Typically, the depressed trench immediately surrounds the innercladding. The depressed trench has a width w_(t) of between about 3 and5 microns (e.g., 4 microns). The refractive index difference Δn_(t)between the depressed trench and the outer cladding is typically betweenabout −15×10⁻³ and −3×10⁻³, more typically between about −10×10⁻³ and−5×10⁻³, such as −6×10⁻³. In this regard, the term “depressed trench” isused to describe a radial portion of an optical fiber that has arefractive index that is substantially less than the refractive index ofthe outer cladding.

In view of the foregoing, a particular exemplary multimode optical fiber(e.g., MaxCap-BB) might include a 50-micron-diameter, graded-indexcentral core defined by an alpha parameter α of about 2.06 and having amaximum refractive index difference Δn₁ with the outer cladding of about14.5×10⁻³ (e.g., Δn₁≈1%). An inner cladding, which contiguouslysurrounds the central core, might have a width w₂ of about 1.5 micronsand a negligible refractive index difference Δn₂ with the outercladding. A depressed trench, which contiguously surrounds the innercladding, might have a width w_(t) of about 4 microns and refractiveindex difference Δn_(t) with the outer cladding of about −6×10⁻³.

This particular, exemplary multimode optical fiber (e.g., Draka'sMaxCap-BB) exhibits, at a wavelength between about 840 nanometers and860 nanometers, a leaky-mode bandwidth (EMB_(leaky)) of about 1,850MHz-km or greater at 300 meters (e.g., for OM3 compliance) and/or aleaky-mode bandwidth (EMB_(leaky)) of about 2,550 MHz-km or greater at400 meters (e.g., for OM4 compliance). As noted, these leaky-modebandwidths satisfy the safe-transmission requirements for OM3-compliantbend-insensitive multimode optical fibers and OM4-compliantbend-insensitive multimode optical fibers, respectively.

In another exemplary embodiment, the present multimode optical fiberexhibits, at a wavelength of about 850 nanometers, a leaky-modebandwidth (EMB_(leaky)) of about 5,000 MHz-km or greater at 300 meters(e.g., between about 7,500 MHz-km and 10,000 MHz-km).

In yet another exemplary embodiment, the present multimode optical fiberexhibits, at a wavelength of about 850 nanometers, a leaky-modebandwidth (EMB_(leaky)) of about 7,500 MHz-km or greater at 400 meters(e.g., 10,000 MHz-km or greater).

The OM3 standard requires, at a wavelength of 850 nanometers, an EMB ofat least 2,000 MHz·km. The OM3 standard assures error-free transmissionsfor a data rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. TheOM4 standard requires, at a wavelength of 850 nanometers, an EMB of atleast 4,700 MHz·km to obtain error-free transmissions for a data rate of10 Gb/s (10 GbE) up to a distance of 400 meters. Each of the OM3 and OM4standards is hereby incorporated by reference in its entirety.

The present optical fibers may facilitate the reduction in overalloptical-fiber diameter. As will be appreciated by those having ordinaryskill in the art, a reduced-diameter optical fiber is cost-effective,requiring less raw material. Moreover, a reduced-diameter optical fiberrequires less deployment space (e.g., within a buffer tube and/or fiberoptic cable), thereby facilitating increased fiber count and/or reducedcable size.

Those having ordinary skill in the art will recognize that an opticalfiber with a primary coating (and an optional secondary coating and/orink layer) typically has an outer diameter of between about 235 micronsand about 265 microns (μm). The component glass fiber itself (i.e., theglass core and surrounding cladding layers) typically has a diameter ofabout 125 microns, such that the total coating thickness is typicallybetween about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fibertypically has an outer diameter of about 125 microns. With respect tothe optical fiber's surrounding coating layers, the primary coatingtypically has an outer diameter of between about 175 microns and about195 microns (i.e., a primary coating thickness of between about 25microns and 35 microns), and the secondary coating typically has anouter diameter of between about 235 microns and about 265 microns (i.e.,a secondary coating thickness of between about 20 microns and 45microns). Optionally, the present optical fiber may include an outermostink layer, which is typically between two and ten microns in thickness.

In one alternative embodiment, an optical fiber may possess a reduceddiameter (e.g., an outermost diameter between about 150 microns and 230microns). In this alternative optical fiber configuration, the thicknessof the primary coating and/or secondary coating is reduced, while thediameter of the component glass fiber is maintained at about 125microns. (Those having ordinary skill in the art will appreciate that,unless otherwise specified, diameter measurements refer to outerdiameters.)

By way of illustration, in such exemplary embodiments, the primarycoating layer may have an outer diameter of between about 135 micronsand about 175 microns (e.g., about 160 microns), typically less than 165microns (e.g., between about 135 microns and 150 microns), and usuallymore than 140 microns (e.g., between about 145 microns and 155 microns,such as about 150 microns).

Moreover, in such exemplary embodiments, the secondary coating layer mayhave an outer diameter of between about 150 microns and about 230microns (e.g., more than about 165 microns, such as 190-210 microns orso), typically between about 180 microns and 200 microns. In otherwords, the total diameter of the optical fiber is reduced to less thanabout 230 microns (e.g., between about 195 microns and 205 microns, andespecially about 200 microns). By way of further illustration, anoptical fiber may employ a secondary coating of about 197 microns at atolerance of +/−5 microns (i.e., a secondary-coating outer diameter ofbetween 192 microns to 202 microns). Typically, the secondary coatingwill retain a thickness of at least about 10 microns (e.g., an opticalfiber having a reduced thickness secondary coating of between 15 micronsand 25 microns).

In another alternative embodiment, the outer diameter of the componentglass fiber may be reduced to less than 125 microns (e.g., between about60 microns and 120 microns), perhaps between about 70 microns and 115microns (e.g., about 80-110 microns). This may be achieved, forinstance, by reducing the thickness of one or more cladding layers. Ascompared with the prior alternative embodiment, (i) the total diameterof the optical fiber may be reduced (i.e., the thickness of the primaryand secondary coatings are maintained in accordance with the prioralternative embodiment) or (ii) the respective thicknesses of theprimary and/or secondary coatings may be increased relative to the prioralternative embodiment (e.g., such that the total diameter of theoptical fiber might be maintained).

By way of illustration, with respect to the former, a component glassfiber having a diameter of between about 90 and 100 microns might becombined with a primary coating layer having an outer diameter ofbetween about 110 microns and 150 microns (e.g., about 125 microns) anda secondary coating layer having an outer diameter of between about 130microns and 190 microns (e.g., about 155 microns). With respect to thelatter, a component glass fiber having a diameter of between about 90and 100 microns might be combined with a primary coating layer having anouter diameter of between about 120 microns and 140 microns (e.g., about130 microns) and a secondary coating layer having an outer diameter ofbetween about 160 microns and 230 microns (e.g., about 195-200 microns).

Reducing the diameter of the component glass fiber might make theresulting optical fiber more susceptible to microbending attenuation.That said, the advantages of further reducing optical-fiber diametermight be worthwhile for some optical-fiber applications.

As noted, the present optical fibers may include one or more coatinglayers (e.g., a primary coating and a secondary coating). At least oneof the coating layers—typically the secondary coating—may be coloredand/or possess other markings to help identify individual fibers.Alternatively, a tertiary ink layer may surround the primary andsecondary coatings.

The present optical fibers may be manufactured by drawing from finalpreforms.

A final preform may be manufactured by providing a primary preform withan outer overcladding layer (i.e., an overcladding process). The outerovercladding layer typically consists of doped or undoped, natural orsynthetic, silica glass. Several methods are available for providing theouter overcladding layer.

In a first exemplary method, the outer overcladding layer may beprovided by depositing and vitrifying natural or synthetic silicaparticles on the outer periphery of the primary preform under theinfluence of heat. Such a process is known, for example, from U.S. Pat.Nos. 5,522,007, 5,194,714, 6,269,663, and 6,202,447, each of which ishereby incorporated by reference in its entirety.

In another exemplary method, a primary preform may be overcladded usinga silica sleeve tube, which may or may not be doped. This sleeve tubemay then be collapsed onto the primary preform.

In yet another exemplary method, an overcladding layer may be appliedvia an Outside Vapor Deposition (OVD) method. Here, a soot layer isfirst deposited on the outer periphery of a primary preform, and thenthe soot layer is vitrified to form glass.

Primary preforms may be manufactured via outside vapor depositiontechniques, such as Outside Vapor Deposition (OVD) and Vapor AxialDeposition (VAD). Alternatively, primary preforms may be manufacturedvia inside deposition techniques in which glass layers are deposited onthe inner surface of a substrate tube of doped or undoped silica glass,such as Modified Chemical Vapor Deposition (MCVD), Furnace ChemicalVapor Deposition (FCVD), and Plasma Chemical Vapor Deposition (PCVD).

By way of example, primary preforms may be manufactured using a PCVDprocess, which can precisely control the central core's gradientrefractive index profile.

A depressed trench, for instance, may be deposited on the inner surfaceof a substrate tube as part of the chemical vapor deposition process.More typically, a depressed trench may be manufactured either (i) byusing a fluorine-doped substrate tube as the starting point of theinternal deposition process for deposition of the gradient refractiveindex central core or (ii) by sleeving a fluorine-doped silica tube overthe gradient refractive index central core, which itself may be producedusing an outside deposition process (e.g., OVD or VAD). Accordingly, acomponent glass fiber manufactured from the resulting preform may have adepressed trench located near the periphery of its central core.

As noted, a primary preform may be manufactured via an inside depositionprocess using a fluorine-doped substrate tube. The resulting tubecontaining the deposited layers may be sleeved by one or more additionalfluorine-doped silica tubes so as to increase the thickness of adepressed trench, or to create a depressed trench having a varyingrefractive index over its width. Although not required, one or moreadditional sleeve tubes (e.g., fluorine-doped substrate tubes) may becollapsed onto the primary preform before an overcladding step iscarried out. The process of sleeving and collapsing is sometimesreferred to as jacketing and may be repeated to build several glasslayers on the outside of the primary preform.

The present optical fibers may be deployed in various structures, suchas those exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosedwithin a buffer tube. For instance, optical fiber may be deployed ineither a single-fiber loose buffer tube or a multi-fiber loose buffertube. With respect to the latter, multiple optical fibers may be bundledor stranded within a buffer tube or other structure. In this regard,within a multi-fiber loose buffer tube, fiber sub-bundles may beseparated with binders (e.g., each fiber sub-bundle is enveloped in abinder). Moreover, fan-out tubing may be installed at the termination ofsuch loose buffer tubes to directly terminate loose buffered opticalfibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermostoptical fiber coating (i.e., tight buffered fiber) or otherwise surroundthe outermost optical-fiber coating or ink layer to provide an exemplaryradial clearance of between about 50 and 100 microns (i.e., a semi-tightbuffered fiber).

With respect to the former tight buffered fiber, the buffering may beformed by coating the optical fiber with a curable composition (e.g., aUV-curable material) or a thermoplastic material. The outer diameter oftight buffer tubes, regardless of whether the buffer tube is formed froma curable or non-curable material, is typically less than about 1,000microns (e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may beincluded between the optical fiber and the buffer tube (e.g., to providea gliding layer).

As will be known by those having ordinary skill in the art, an exemplarybuffer tube enclosing optical fibers as disclosed herein may be formedof polyolefins (e.g., polyethylene or polypropylene), includingfluorinated polyolefins, polyesters (e.g., polybutylene terephthalate),polyamides (e.g., nylon), as well as other polymeric materials andblends. In general, a buffer tube may be formed of one or more layers.The layers may be homogeneous or include mixtures or blends of variousmaterials within each layer.

In this context, the buffer tube may be extruded (e.g., an extrudedpolymeric material) or pultruded (e.g., a pultruded, fiber-reinforcedplastic). By way of example, the buffer tube may include a material toprovide high temperature and chemical resistance (e.g., an aromaticmaterial or polysulfone material).

Although buffer tubes typically have a circular cross section, buffertubes alternatively may have an irregular or non-circular shape (e.g.,an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply besurrounded by an outer protective sheath or encapsulated within a sealedmetal tube. In either structure, no intermediate buffer tube isnecessarily required.

Multiple optical fibers as disclosed herein may be sandwiched,encapsulated, and/or edge bonded to form an optical fiber ribbon.Optical fiber ribbons can be divisible into subunits (e.g., atwelve-fiber ribbon that is splittable into six-fiber subunits).Moreover, a plurality of such optical fiber ribbons may be aggregated toform a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or aribbon stack in which the uppermost and lowermost optical fiber ribbonshave fewer optical fibers than those toward the center of the stack.This construction may be useful to increase the density of opticalelements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmissionelements in buffer tubes or cables, subject to other constraints (e.g.,cable or mid-span attenuation). The optical elements themselves may bedesigned for increased packing density. For example, the optical fibermay possess modified properties, such as improved refractive-indexprofile, core or cladding dimensions, or primary-coating thicknessand/or modulus, to improve microbending and macrobendingcharacteristics.

By way of example, a rectangular ribbon stack may be formed with orwithout a central twist (i.e., a “primary twist”). Those having ordinaryskill in the art will appreciate that a ribbon stack is typicallymanufactured with rotational twist to allow the tube or cable to bendwithout placing excessive mechanical stress on the optical fibers duringwinding, installation, and use. In a structural variation, a twisted (oruntwisted) rectangular ribbon stack may be further formed into acoil-like configuration (e.g., a helix) or a wave-like configuration(e.g., a sinusoid). In other words, the ribbon stack may possess regular“secondary” deformations.

As will be known to those having ordinary skill in the art, such opticalfiber ribbons may be positioned within a buffer tube or othersurrounding structure, such as a buffer-tube-free cable. Subject tocertain restraints (e.g., attenuation), it is desirable to increase thedensity of elements such as optical fibers or optical fiber ribbonswithin buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be positioned externally adjacent to and strandedaround a central strength member. This stranding can be accomplishedhelically in one direction, known as “S” or “Z” stranding, or viaReverse Oscillated Lay stranding, known as “S-Z” stranding. Strandingabout the central strength member reduces optical fiber strain whencable strain occurs during installation and use.

Those having ordinary skill in the art will understand the benefit ofminimizing fiber strain for both tensile cable strain and longitudinalcompressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur duringinstallation, the cable will become longer while the optical fibers canmigrate closer to the cable's neutral axis to reduce, if not eliminate,the strain being translated to the optical fibers. With respect tolongitudinal compressive strain, which may occur at low operatingtemperatures due to shrinkage of the cable components, the opticalfibers will migrate farther away from the cable's neutral axis toreduce, if not eliminate, the compressive strain being translated to theoptical fibers.

In a variation, two or more substantially concentric layers of buffertubes may be positioned around a central strength member. In a furthervariation, multiple stranding elements (e.g., multiple buffer tubesstranded around a strength member) may themselves be stranded aroundeach other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers(e.g., loose or ribbonized fibers) may be simply placed externallyadjacent to the central strength member (i.e., the buffer tubes are notintentionally stranded or arranged around the central strength member ina particular manner and run substantially parallel to the centralstrength member).

Alternatively still, the present optical fibers may be positioned withina central buffer tube (i.e., the central buffer tube cable has a centralbuffer tube rather than a central strength member). Such a centralbuffer tube cable may position strength members elsewhere. For instance,metallic or non-metallic (e.g., GRP) strength members may be positionedwithin the cable sheath itself, and/or one or more layers ofhigh-strength yarns (e.g., aramid or non-aramid yarns) may be positionedparallel to or wrapped (e.g., contrahelically) around the central buffertube (i.e., within the cable's interior space). As will be understood bythose having ordinary skill in the art, such strength yarns providetensile strength to fiber optic cables. Likewise, strength members canbe included within the buffer tube's casing.

Strength yarns may be coated with a lubricant (e.g., fluoropolymers),which may reduce unwanted attenuation in fiber optic cables (e.g.,rectangular, flat ribbon cables or round, loose tube cables) that aresubjected to relatively tight bends (i.e., a low bend radius). Moreover,the presence of a lubricant on strength yarns (e.g., aramid strengthyarns) may facilitate removal of the cable jacketing by reducingunwanted bonding between the strength yarns and the surrounding cablejacket.

In other embodiments, the optical fibers may be placed within a slottedcore cable. In a slotted core cable, optical fibers, individually or asa fiber ribbon, may be placed within pre-shaped helical grooves (i.e.,channels) on the surface of a central strength member, thereby forming aslotted core unit. The slotted core unit may be enclosed by a buffertube. One or more of such slotted core units may be placed within aslotted core cable. For example, a plurality of slotted core units maybe helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitubecable design, whereby the optical fibers are stranded around themselveswithin a large multi-fiber loose buffer tube rather than around acentral strength member. In other words, the large multi-fiber loosebuffer tube is centrally positioned within the maxitube cable. Forexample, such maxitube cables may be deployed in optical ground wires(OPGW).

In another cabling embodiment, multiple buffer tubes may be strandedaround themselves without the presence of a central member. Thesestranded buffer tubes may be surrounded by a protective tube. Theprotective tube may serve as the outer casing of the fiber optic cableor may be further surrounded by an outer sheath. The protective tube mayeither tightly surround or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additionalelements may be included within a cable core. For example, copper cablesor other active, transmission elements may be stranded or otherwisebundled within the cable sheath. Passive elements may also be placedwithin the cable core, such as between the interior walls of the buffertubes and the enclosed optical fibers. Alternatively and by way ofexample, passive elements may be placed outside the buffer tubes betweenthe respective exterior walls of the buffer tubes and the interior wallof the cable jacket, or within the interior space of a buffer-tube-freecable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or othermaterials containing water-swellable material and/or coated withwater-swellable materials (e.g., including super absorbent polymers(SAPs), such as SAP powder) may be employed to provide water blockingand/or to couple the optical fibers to the surrounding buffer tubeand/or cable jacketing (e.g., via adhesion, friction, and/orcompression). Exemplary water-swellable elements are disclosed incommonly assigned U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube, which ishereby incorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive,such as a silicone acrylate cross-linked by exposure to actinicradiation) may be provided on one or more passive elements (e.g.,water-swellable material) to bond the elements to the buffer tube. Anadhesive material may also be used to bond the water-swellable elementto optical fibers within the buffer tube. Exemplary arrangements of suchelements are disclosed in commonly assigned U.S. Pat. No. 7,599,589 fora Gel-Free Buffer Tube with Adhesively Coupled Optical Element, which ishereby incorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain athixotropic composition (e.g., grease or grease-like gels) between theoptical fibers and the interior walls of the buffer tubes. For example,filling the free space inside a buffer tube with water-blocking,petroleum-based filling grease helps to block the ingress of water.Further, the thixotropic filling grease mechanically (i.e., viscously)couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, therebyhindering connection and splicing operations. Thus, the present opticalfibers may be deployed in dry cable structures (i.e., grease-free buffertubes).

Exemplary buffer tube structures that are free from thixotropic fillinggreases are disclosed in commonly assigned U.S. Pat. No. 7,724,998 for aCoupling Composition for Optical Fiber Cables (Parris et al.), which ishereby incorporated by reference in its entirety. Such buffer tubesemploy coupling compositions formed from a blend of high-molecularweight elastomeric polymers (e.g., about 35 weight percent or less) andoils (e.g., about 65 weight percent or more) that flow at lowtemperatures. Unlike thixotropic filling greases, the couplingcomposition (e.g., employed as a cohesive gel or foam) is typically dryand, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cableenclosing optical fibers as disclosed herein may have a sheath formedfrom various materials in various designs. Cable sheathing may be formedfrom polymeric materials such as, for example, polyethylene,polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon),polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylenepropylene, polyvinyl fluoride, or polyvinylidene difluoride), andethylene vinyl acetate. The sheath and/or buffer tube materials may alsocontain other additives, such as nucleating agents, flame-retardants,smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectricmaterial (e.g., non-conducting polymers), with or without supplementalstructural components that may be used to improve the protection (e.g.,from rodents) and strength provided by the cable sheath. For example,one or more layers of metallic (e.g., steel) tape, along with one ormore dielectric jackets, may form the cable sheathing. Metallic orfiberglass reinforcing rods (e.g., GRP) may also be incorporated intothe sheath. In addition, aramid, fiberglass, or polyester yarns may beemployed under the various sheath materials (e.g., between the cablesheath and the cable core), and/or ripcords may be positioned, forexample, within the cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have acircular cross section, but cable sheaths alternatively may have anirregular or non-circular shape (e.g., an oval, trapezoidal, or flatcross-section).

By way of example, the present optical fiber may be incorporated intosingle-fiber drop cables, such as those employed for Multiple DwellingUnit (MDU) applications. In such deployments, the cable jacketing mustexhibit crush resistance, abrasion resistance, puncture resistance,thermal stability, and fire resistance as required by building codes. Anexemplary material for such cable jackets is thermally stable,flame-retardant polyurethane (PUR), which mechanically protects theoptical fibers yet is sufficiently flexible to facilitate easy MDUinstallations. Alternatively, a flame-retardant polyolefin or polyvinylchloride sheath may be used.

In general, and as will be known to those having ordinary skill in theart, a strength member is typically in the form of a rod orbraided/helically wound wires or fibers, though other configurationswill be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may bevariously deployed, including as drop cables, distribution cables,feeder cables, trunk cables, and stub cables, each of which may havevarying operational requirements (e.g., temperature range, crushresistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts,plenums, or risers. By way of example, an optical fiber cable may beinstalled in an existing duct or microduct by pulling or blowing (e.g.,using compressed air). An exemplary cable installation method isdisclosed in commonly assigned U.S. Pat. No. 7,574,095 for aCommunication Cable Assembly and Installation Method (Lock et al.), andU.S. Pat. No. 7,665,902 for a Modified Pre-Ferrulized CommunicationCable Assembly and Installation Method (Griffioen et al.), each of whichis incorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be stranded (e.g., around a central strengthmember). In such configurations, an optical fiber cable's protectiveouter sheath may have a textured outer surface that periodically varieslengthwise along the cable in a manner that replicates the strandedshape of the underlying buffer tubes. The textured profile of theprotective outer sheath can improve the blowing performance of theoptical fiber cable. The textured surface reduces the contact surfacebetween the cable and the duct or microduct and increases the frictionbetween the blowing medium (e.g., air) and the cable. The protectiveouter sheath may be made of a low coefficient-of-friction material,which can facilitate blown installation. Moreover, the protective outersheath can be provided with a lubricant to further facilitate blowninstallation.

In general, to achieve satisfactory long-distance blowing performance(e.g., between about 3,000 to 5,000 feet or more), the outer cablediameter of an optical fiber cable should be no more than about 70 to 80percent of the duct's or microduct's inner diameter.

Compressed air may also be used to install optical fibers in an airblown fiber system. In an air blown fiber system, a network of unfilledcables or microducts is installed prior to the installation of opticalfibers. Optical fibers may subsequently be blown into the installedcables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the groundor, as an aerial cable, suspended from a pole or pylon. An aerial cablemay be self-supporting, or secured or lashed to a support (e.g.,messenger wire or another cable). Exemplary aerial fiber optic cablesinclude overhead ground wires (OPGW), all-dielectric self-supportingcables (ADSS), all dielectric lash cables (AD-Lash), and figure-eightcables, each of which is well understood by those having ordinary skillin the art. Figure-eight cables and other designs can be directly buriedor installed into ducts, and may optionally include a toning element,such as a metallic wire, so that they can be found with a metaldetector.

In addition, although the optical fibers may be further protected by anouter cable sheath, the optical fiber itself may be further reinforcedso that the optical fiber may be included within a breakout cable, whichallows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmissionsystem, connections are required at various points in the network.Optical fiber connections are typically made by fusion splicing,mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the optical fiber endseither in the field (e.g., at the network location) or in a factoryprior to installation into the network. The ends of the connectors aremated in the field in order to connect the optical fibers together orconnect the optical fibers to the passive or active components. Forexample, certain optical fiber cable assemblies (e.g., furcationassemblies) can separate and convey individual optical fibers from amultiple optical fiber cable to connectors in a protective manner.

The deployment of such optical fiber cables may include supplementalequipment, which itself may employ the present optical fiber aspreviously disclosed. For instance, an amplifier may be included toimprove optical signals. Dispersion compensating modules may beinstalled to reduce the effects of chromatic dispersion and polarizationmode dispersion. Splice boxes, pedestals, and distribution frames, whichmay be protected by an enclosure, may likewise be included. Additionalelements include, for example, remote terminal switches, optical networkunits, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use ina communication system (e.g., networking or telecommunications). Acommunication system may include fiber optic cable architecture such asfiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure(FTTE), fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), andfiber-to-the-home (FTTH), as well as long-haul or metro architecture.Moreover, an optical module or a storage box that includes a housing mayreceive a wound portion of the optical fiber disclosed herein. By way ofexample, the optical fiber may be wound around a bending radius of lessthan about 15 millimeters (e.g., 10 millimeters or less, such as about 5millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications,including, without limitation, fiber optic sensors or illuminationapplications (e.g., lighting).

The present optical fibers may include Fiber Bragg Grating (FBG). Aswill be known by those having ordinary skill in the art, FBG is aperiodic or aperiodic variation in the refractive index of an opticalfiber core and/or cladding. This variation in the refractive indexresults in a range of wavelengths (e.g., a narrow range) being reflectedrather than transmitted, with maximum reflectivity occurring at theBragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber byexposing the optical fiber to an intense source of ultraviolet light(e.g., a UV laser). In this respect, UV photons may have enough energyto break molecular bonds within an optical fiber, which alters thestructure of the optical fiber, thereby increasing the optical fiber'srefractive index. Moreover, dopants (e.g., boron or germanium) and/orhydrogen loading can be employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation ofFBG, the coating may be removed. Alternatively, coatings that aretransparent at the particular UV wavelengths (e.g., the UV wavelengthsemitted by a UV laser to write FBG) may be employed to render coatingremoval unnecessary. In addition, silicone, polyimide, acrylate, or PFCBcoatings, for instance, may be employed for high-temperatureapplications.

A particular FBG pattern may be created by employing (i) a photomaskplaced between the UV light source and the optical fiber, (ii)interference between multiple UV light beams, which interfere with eachother in accordance with the desired FBG pattern (e.g., a uniform,chirped, or titled pattern), or (iii) a narrow UV light beam forcreating individual variations. The FBG structure may have, for example,a uniform positive-only index change, a Gaussian-apodized index change,a raised-cosine-apodized index change, or a discrete phase-shift indexchange. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensingapplications (e.g., for detecting vibration, temperature, pressure,moisture, or movement). In this respect, changes in the optical fiber(e.g., a change in temperature) result in a shift in the Braggwavelength, which is measured by a sensor. FBG may be used to identify aparticular optical fiber (e.g., if the optical fiber is broken intopieces).

Fiber Bragg Grating may also be used in various active or passivecommunication components (e.g., wavelength-selective filters,multiplexers, demultiplexers, Mach-Zehnder interferometers, distributedBragg reflector lasers, pump/laser stabilizers, and supervisorychannels).

To supplement the present disclosure, this application incorporatesentirely by reference the following commonly assigned patents, patentapplication publications, and patent applications: U.S. Pat. No.4,838,643 for a Single Mode Bend Insensitive Fiber for Use in FiberOptic Guidance Applications (Hodges et al.); U.S. Pat. No. 7,623,747 fora Single Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No.7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S.Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (deMontmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic DispersionCompensating Fiber (Bigot-Astruc et al.); U.S. Pat. No. 7,526,177 for aFluorine-Doped Optical Fiber (Matthijsse et al.); U.S. Pat. No.7,555,186 for an Optical Fiber (Flammer et al.); U.S. Pat. No. 8,055,111for a Dispersion-Shifted Optical Fiber (Sillard et al.); U.S. Pat. No.8,041,172 for a Transmission Optical Fiber Having Large Effective Area(Sillard et al.); International Patent Application Publication No. WO2009/062131 A1 for a Microbend-Resistant Optical Fiber (Overton); U.S.Pat. No. 8,265,442 for a Microbend-Resistant Optical Fiber (Overton);U.S. Pat. No. 8,145,025 for a Single-Mode Optical Fiber Having ReducedBending Losses (de Montmorillon et al.); U.S. Pat. No. 7,889,960 for aBend-Insensitive Single-Mode Optical Fiber (de Montmorillon et al.);U.S. Patent Application Publication No. US2010/0021170 A1 for aWavelength Multiplexed Optical System with Multimode Optical Fibers(Lumineau et al.); U.S. Pat. No. 7,995,888 for a Multimode OpticalFibers (Gholami et al.); U.S. Patent Application Publication No.US2010/0119202 A1 for a Reduced-Diameter Optical Fiber (Overton); U.S.Patent Application Publication No. US2010/0142969 A1 for a MultimodeOptical System (Gholami et al.); U.S. Pat. No. 8,259,389 for anAmplifying Optical Fiber and Method of Manufacturing (Pastouret et al.);U.S. Patent Application Publication No. US2010/0135627 A1 for anAmplifying Optical Fiber and Production Method (Pastouret et al.); U.S.Patent Application Publication No. US2010/0142033 for an IonizingRadiation-Resistant Optical Fiber Amplifier (Regnier et al.); U.S. Pat.No. 8,274,647 for a Method of Classifying a Graded-Index MultimodeOptical Fiber (Gholami et al.); U.S. Patent Application Publication No.US2010/0189397 A1 for a Single-Mode Optical Fiber (Richard et al.); U.S.Patent Application Publication No. US2010/0189399 A1 for a Single-ModeOptical Fiber Having an Enlarged Effective Area (Sillard et al.); U.S.Patent Application Publication No. US2010/0189400 A1 for a Single-ModeOptical Fiber (Sillard et al.); U.S. Patent Application Publication No.US2010/0214649 A1 for an Optical Fiber Amplifier Having Nanostructures(Burow et al.); U.S. Pat. No. 8,009,950 for a Multimode Fiber (Molin etal.); U.S. Patent Application Publication No. US2010/0310218 A1 for aLarge Bandwidth Multimode Optical Fiber Having a Reduced Cladding Effect(Molin et al.); U.S. Patent Application Publication No. US2011/0058781A1 for a Multimode Optical Fiber Having Improved Bending Losses (Molinet al.); U.S. Patent Application Publication No. US2011/0064367 A1 for aMultimode Optical Fiber (Molin et al.); U.S. Patent ApplicationPublication No. US2011/0069724 A1 for an Optical Fiber for Sum-FrequencyGeneration (Richard et al.); U.S. Patent Application Publication No.US2011/0116160 A1 for a Rare-Earth-Doped Optical Fiber Having SmallNumerical Aperture (Boivin et al.); U.S. Pat. No. 8,280,213 for aHigh-Bandwidth, Multimode Optical Fiber with Reduced Cladding Effect(Molin et al.); U.S. Patent Application Publication No. US2011/0123162A1 for a High-Bandwidth, Dual-Trench-Assisted Multimode Optical Fiber(Molin et al.); U.S. Patent Application Publication No. US2011/0135262A1 for a Multimode Optical Fiber with Low Bending Losses and ReducedCladding Effect (Molin et al.); U.S. Patent Application Publication No.US2011/0135263 A1 for a High-Bandwidth Multimode Optical Fiber HavingReduced Bending Losses (Molin et al.); U.S. Patent ApplicationPublication No. US2011/0188826 A1 for a Non-Zero Dispersion ShiftedOptical Fiber Having a Large Effective Area (Sillard et al.); U.S.Patent Application Publication No. US2011/0188823 A1 for a Non-ZeroDispersion Shifted Optical Fiber Having a Short Cutoff Wavelength(Sillard et al.); U.S. Patent Application Publication No. 2011/0217012A1 for a Broad-Bandwidth Multimode Optical Fiber Having Reduced BendingLosses (Bigot-Astruc et al.); U.S. Patent Application Publication No.2011/0229101 A1 for a Single-Mode Optical Fiber (de Montmorillon etal.); U.S. Patent Application Publication No. 2012/0051703 A1 for aSingle-Mode Optical Fiber (Bigot-Astruc et al.); U.S. Patent ApplicationPublication No. 2012/0040184 A1 for a Method of Fabricating an OpticalFiber Preform (de Montmorillon et al.); U.S. Patent ApplicationPublication No. 2012/0092651 A1 for a Multimode Optical FiberInsensitive to Bending Losses (Molin et al.); U.S. Patent ApplicationPublication No. 2012/0134376 A1 for a Radiation-Insensitive OpticalFiber Doped with Rare Earths (Burov et al.); U.S. Patent ApplicationPublication No. 2012/0148206 A1 for a Rare-Earth-Doped Optical Fiber(Boivin et al.); U.S. Patent Application Publication No. 2012/0195549 A1for a Broad-Bandwidth Optical Fiber (Molin et al.); U.S. PatentApplication Publication No. 2012/0195561 A1 for a Multimode OpticalFiber (Molin et al.); U.S. Patent Application Publication No.2012/00224254 A1 for a Rare-Earth-Doped Amplifying Optical Fiber (Burovet al.); U.S. patent application Ser. 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To supplement the present disclosure, this application furtherincorporates entirely by reference the following commonly assignedpatents, patent application publications, and patent applications: U.S.Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubesfor Optical Fiber Cables and Method for Making the Same; U.S. Pat. No.5,717,805 for Stress Concentrations in an Optical Fiber Ribbon toFacilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,911,023 forPolyolefin Materials Suitable for Optical Fiber Cable Components; U.S.Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbonto Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No.6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No.6,066,397 for Polypropylene Filler Rods for Optical Fiber CommunicationsCables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon andMethod for Making the Same; U.S. Pat. No. 6,085,009 for Water BlockingGels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes andCables Made Therewith; U.S. Pat. No. 6,215,931 for FlexibleThermoplastic Polyolefin Elastomers for Buffering Transmission Elementsin a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method forAccessing Optical Fibers in the Midspan Region of an Optical FiberCable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbonand Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method forAccessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section;U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix MaterialHaving Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for anOptical Fiber Having Water Swellable Material for Identifying Groupingof Fiber Groups; U.S. Pat. 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In the specification and/or figures, typical embodiments of theinvention have been disclosed. The present invention is not limited tosuch exemplary embodiments. The use of the term “and/or” includes anyand all combinations of one or more of the associated listed items. Thefigures are schematic representations and so are not necessarily drawnto scale. Unless otherwise noted, specific terms have been used in ageneric and descriptive sense and not for purposes of limitation.

The invention claimed is:
 1. A multimode optical fiber, comprising: acentral core surrounded by an outer cladding, said central core havingan outer radius r₁ and an alpha-index profile with respect to said outercladding; an inner cladding positioned between said central core andsaid outer cladding, said inner cladding having (i) an outer radius r₂,(ii) a width w₂, and (iii) a refractive index difference Δn₂ withrespect to said outer cladding; and a depressed trench positionedbetween said inner cladding and said outer cladding, said depressedtrench having (i) an outer radius r_(t), (ii) a width w_(t), and (iii) anegative refractive index difference Δn_(t) with respect to said outercladding; wherein said depressed trench has a negative volume V definedby the expression V=1000×w_(t)×Δn_(t), said depressed trench's negativevolume V having an absolute value of greater than 20 microns; wherein,at a wavelength between about 840 nanometers and 860 nanometers, themultimode optical fiber exhibits a leaky-mode bandwidth (EMB_(leaky)) ofabout 2,550 MHz-km or greater at 400 meters.
 2. The multimode opticalfiber according to claim 1, wherein said inner cladding immediatelysurrounds said central core and said depressed trench immediatelysurrounds said inner cladding.
 3. The multimode optical fiber accordingto claim 1, wherein said central core's outer radius r₁ is 25microns±1.5 microns.
 4. A multimode optical fiber, comprising: a centralcore surrounded by an outer cladding, said central core having an outerradius r₁ and an alpha-index profile with respect to said outercladding; an inner cladding positioned between said central core andsaid outer cladding, said inner cladding having (i) an outer radius r₂,(ii) a width w₂, and (iii) a refractive index difference Δn₂ withrespect to said outer cladding; and a depressed trench positionedbetween said inner cladding and said outer cladding, said depressedtrench having (i) an outer radius r_(t), (ii) a width w_(t), and (iii) anegative refractive index difference Δn_(t) with respect to said outercladding; wherein said depressed trench has a negative volume V definedby the expression V=1000×w_(t)×Δn_(t), said depressed trench's negativevolume V having an absolute value of greater than 20 microns; wherein,at a wavelength between about 840 nanometers and 860 nanometers, themultimode optical fiber exhibits a leaky-mode bandwidth (EMB_(leaky)) ofabout 1,850 MHz-km or greater at 300 meters.
 5. The multimode opticalfiber according to claim 4, wherein said inner cladding immediatelysurrounds said central core and said depressed trench immediatelysurrounds said inner cladding.
 6. The multimode optical fiber accordingto claim 4, wherein said central core's outer radius r₁ is 25microns±1.5 microns.
 7. The multimode optical fiber according to claim4, wherein said inner cladding's width w₂ is between about 1 micron and2 microns.
 8. The multimode optical fiber according to claim 4, whereinsaid inner cladding's refractive index difference Δn₂ is between about−0.5×10⁻³ and 0.5×10⁻³.
 9. The multimode optical fiber according toclaim 4, wherein said inner cladding's refractive index difference Δn₂is between about −0.05×10⁻³ and 0.05×10⁻³.
 10. The multimode opticalfiber according to claim 4, wherein said depressed trench's refractiveindex difference Δn_(t) is between about −15×10⁻³ and −3×10⁻³.
 11. Themultimode optical fiber according to claim 4, wherein said depressedtrench's width w_(t) is between about 3 microns and 5 microns.
 12. Themultimode optical fiber according to claim 4, wherein said depressedtrench has a volume V of between about −30 microns and −40 microns asdefined by the expression V=1000×w_(t)×Δn_(t).
 13. The multimode opticalfiber according to claim 4, wherein at a wavelength of 850 nanometers,the multimode optical fiber has bending losses of (i) less than about0.4 dB for two turns around a bend radius of 7.5 millimeters and (ii)less than about 1 dB for two turns around a bend radius of 5millimeters.
 14. The multimode optical fiber according to claim 4,wherein, for two turns around a bend radius of 7.5 millimeters at awavelength of 850 nanometers, the multimode optical fiber has bendinglosses of less than about 0.2 dB.
 15. The multimode optical fiberaccording to claim 4, wherein, for two turns around a bend radius of 5millimeters at a wavelength of 850 nanometers, the multimode opticalfiber has bending losses of less than about 0.3 dB.
 16. The multimodeoptical fiber according to claim 4, wherein, with respect to said outercladding, said central core has a maximum refractive index differenceΔn₂ of between about 11×10⁻³ and 16×10⁻³.
 17. The multimode opticalfiber according to claim 4, wherein said central core's alpha-indexprofile has an alpha parameter α of between about 2.04 and 2.10.
 18. Themultimode optical fiber according to claim 4, wherein said innercladding's width w₂ is less than about 5 microns.
 19. The multimodeoptical fiber according to claim 1, wherein, with respect to said outercladding, said central core has a maximum refractive index differenceΔn₁ of between about 11×10⁻³ and 16×10⁻³.
 20. The multimode opticalfiber according to claim 4, wherein said central core's alpha-indexprofile has an alpha parameter α of between about 2.04 and 2.10.
 21. Themultimode optical fiber according to claim 4, wherein said innercladding's width w₂ is less than about 5 microns.
 22. The multimodeoptical fiber according to claim 4, wherein said inner cladding's widthw₂ is between about 1 micron and 2 microns.
 23. The multimode opticalfiber according to claim 4, wherein said inner cladding's refractiveindex difference Δn₂ is between about −0.5×10⁻³ and 0.5×10⁻³.
 24. Themultimode optical fiber according to claim 4, wherein said innercladding's refractive index difference Δn₂ is between about −0.05×10⁻³and 0.05×10⁻³.
 25. The multimode optical fiber according to claim 4,wherein said depressed trench's refractive index difference Δn_(t) isbetween about −15×10⁻³ and −3×10⁻³.
 26. The multimode optical fiberaccording to claim 4, wherein said depressed trench's width w_(t) isbetween about 3 microns and 5 microns.
 27. The multimode optical fiberaccording to claim 4, wherein said depressed trench has a volume V ofbetween about −30 microns and −40 microns as defined by the expressionV=1000×w_(t)×Δn_(t).
 28. The multimode optical fiber according to claim4, wherein at a wavelength of 850 nanometers, the multimode opticalfiber has bending losses of (i) less than about 0.4 dB for two turnsaround a bend radius of 7.5 millimeters and (ii) less than about 1 dBfor two turns around a bend radius of 5 millimeters.
 29. The multimodeoptical fiber according to claim 5, wherein, at a wavelength betweenabout 840 nanometers and 860 nanometers, the multimode optical fiberexhibits a leaky-mode bandwidth (EMB_(leaky)) of about 3,880 MHz-km orgreater at 400 meters.
 30. The multimode optical fiber according toclaim 4, wherein, for two turns around a bend radius of 7.5 millimetersat a wavelength of 850 nanometers, the multimode optical fiber hasbending losses of less than about 0.2 dB.
 31. The multimode opticalfiber according to claim 4, wherein, for two turns around a bend radiusof 5 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 0.3 dB.
 32. Amultimode optical fiber, comprising: a central core surrounded by anouter cladding, said central core having an outer radius r₁ and analpha-index profile with respect to said outer cladding; an innercladding positioned between said central core and said outer cladding,said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and(iii) a refractive index difference Δn₂ with respect to said outercladding; and a depressed trench positioned between said inner claddingand said outer cladding, said depressed trench having (i) an outerradius r_(t), (ii) a width w_(t), and (iii) a negative refractive indexdifference Δn_(t) with respect to said outer cladding; wherein saiddepressed trench has a negative volume V defined by the expressionV=1000×w_(t)×Δn_(t), said depressed trench's negative volume V having anabsolute value of greater than 20 microns; wherein, at a wavelength ofabout 850 nanometers, the multimode optical fiber exhibits a leaky-modebandwidth (EMB_(leaky)) of (i) about 1,850 MHz-km or greater at 300meters and (ii) about 2,550 MHz-km or greater at 400 meters.
 33. Themultimode optical fiber according to claim 32, wherein said innercladding immediately surrounds said central core and said depressedtrench immediately surrounds said inner cladding.
 34. The multimodeoptical fiber according to claim 32, wherein, with respect to said outercladding, said central core has a maximum refractive index differenceΔn₂ of between about 12×10⁻³ and 15×10⁻³.
 35. The multimode opticalfiber according to claim 32, wherein, wherein (i) said central core'souter radius r₁ is 25 microns±1.5 microns and (ii) said central core'salpha-index profile has an alpha parameter α of between about 1.9 and2.1.
 36. The multimode optical fiber according to claim 32, wherein,wherein said inner cladding's width w₂ is between about 1 micron and 3microns.
 37. The multimode optical fiber according to claim 32, whereinsaid inner cladding's refractive index difference Δn₂ is between about−0.2×10⁻³ and 0.2×10⁻³.
 38. The multimode optical fiber according toclaim 32, wherein said depressed trench's refractive index differenceΔn_(t) is between about −10×10⁻³ and −5×10⁻³.
 39. The multimode opticalfiber according to claim 32, wherein said depressed trench's negativevolume V has an absolute value of greater than 20 microns and less than40 microns.
 40. The multimode optical fiber according to claim 32,wherein, at a wavelength of 850 nanometers, the multimode optical fiberhas bending losses of (i) less than about 0.2 dB for two turns around abend radius of 7.5 millimeters and (ii) less than about 0.3 dB for twoturns around a bend radius of 5 millimeters.
 41. An optical-fiber systemcomprising the multimode optical fiber according to claim 32.